As generally known, the speed of a direct current motor drive may be controlled by changing either the counter E.M.F. of the motor, or the flux. The present invention, when applied to DC motor speed regulation, utilizes armature voltage for field flux control.
One conventional mode of controlling the speed of a DC motor is to control motor terminal voltage with the motor field excitation being kept fixed and the required amount of power of the motor being supplied by a direct current generator.
Another conventional mode of controlling the speed of a motor is normally to operate the motor at a lower speed level corresponding to base speed and to increase the speed above such level by decreasing the field current so as to decrease the counter E.M.F. and increase the armature current, thus the speed.
A speed regulating system of a DC motor can be of the multi-loop type, including an armature current regulating loop and a speed regulating loop. The speed loop is operative in response to a speed reference signal and a speed signal from the tachometer and includes a speed controller for fixing the level of speed to be reached, or maintained. The current loop operates with the armature current through a current transducer and a current controller in order to keep the armature current between strict limits.
A speed regulating system includes at least two such regulating loops, and proper control operation requires that each loop be effective in controlling the system independently of the other. However, in such systems operation of one loop is controlled by summing up the input signal from one loop with the output signal from the other loop. In other words one loop is outside the other. Thus the inner feedback loop may be an armature current loop, or a field current loop. In such instances, the response of the system is limited by the windup characteristic of the outside controller, which may be the speed controller, or even two controllers may cause winding up for instance by the speed and the armature current controllers.
Windup occurs in an outside controller when its output voltage signal continues to change while the inner controller has reached its limit value, e.g. has become saturated. If the outside controller which is winding up contains energy storage elements such as an integrator, unless the stored energy is discharged the outside controller will be unable to follow the controlling events, and when the inner controller recovers from saturation proper control by the system cannot resume. This is a particularly critical situation when the DC motor is controlled by a field exciter working through a large field time delay. This is the case with the generator field exciter when a generator is coupled to the motor, or with the motor field exciter when the motor is supplied from the DC power supply. In such situation, the windup voltage of the speed controller, which represents armature current loop reference, causes the armature current to become too high, thus causing the speed regulator to overshoot. The time required to bring the outside controller back into controlling range causes an over-correction in the regulator system. If the over-correction is excessive, the regulator will automatically force an under-correction to be made thus causing the controller to windup in the opposite direction. As a result oscillation can occur, which oscillation may even be sustained. While a regulator system can be stable for small disturbances, it may break into oscillations for larger disturbances as a result of controller windup, especially with the field time delays encountered in practice which are of the order of 0.5 seconds, or more. This problem can be minimized by forcing the field on the field exciter (associated with the generator or motor depending on the particular regulating system) by providing a larger exciter saturation voltage or by slowing down the outside regulating loop sufficiently to prevent windup of the outside controller. On the one hand increasing the field exciter maximum voltage is an expensive solution, and usually not a practical one. On the other hand, slowing down the outside regulator loop is not an ideal solution either. If the regulators are made slow so that oscillations of the system due to controller windup no longer occur, the overall performance of the DC motor is reduced. For instance in a steel mill this represents a sacrifice in tons of production per hour. The millstand speed regulators as a result of the limiting factor introduced in the regulator response no longer react to small disturbances. Typically when the mill is being threaded the small disturbances are not corrected and strip breakage or "off" gauge strip, occur more often than not, because most load disturbances in the drive system are small while the regulator response to such disturbances is not limited by the field exciter saturation voltage. Thus, slowing down of the speed regulator response is not the right approach to the problem.
Reference can be made to U.S. Pat. No. 3,508,132 of R. S. Peterson entitled "Power Peak Circuiting Control for Direct Current Drives" issued on Apr. 21, 1970. This patent discloses a multi-loop speed regulating system for a direct current motor drive of the hoist type. The regulating system described includes (1) an inner armature current loop comprising a current transducer and a current controller providing control signal for generating a field current to a generator coupled to the D.C. motor drive (2) an outside speed control loop including a speed controller responsive to a reference signal and to the tachometer signal for generating a speed control signal to be summed up with the current transducer signal at the input of the current controller.
The excess speed developed in the system of the above-mentioned patent is used to check speed control. The present invention instead, bears upon a multi-loop regulating system in which excess current is used to check speed control.